Zeolites are crystalline aluminosilicate compositions that are microporous and that are formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and their ability to adsorb and reversibly desorb an adsorbed phase that is dispersed throughout the internal voids of the crystal without significantly displacing any atoms that make up the permanent zeolite crystal structure.
Among other uses, zeolites can be used to make an adsorbent material. In adsorbent materials, zeolites can separate components of either multi-component gas mixtures or liquid mixtures. It's generally understood that the presence of an inert or non-reactive zeolite (“contaminant zeolite”) can often diminish the adsorption performance of certain zeolites. Conventionally, however, the presence of some relatively low, but still tolerable, concentration of a contaminant zeolite has been viewed as commercially acceptable since it's generally considered to produce an insignificant loss or drop in the finished adsorbent's performance. Accordingly, there's a diminishing point of return in further reducing contaminant zeolite levels. So conventionally, it's been considered more cost-effective to leave contaminant zeolite(s) mixed with the active zeolite, than to remove or further reduce the contaminant zeolite in light of its related adsorbent's projected performance.
Accordingly, there is a need for a zeolite with improved purity, more particularly an X type zeolite that can have a more beneficial effect on the zeolite's process performance than expected for the extent to which the contaminant zeolite content is either further reduced, beyond customary levels, or fully removed.
Despite this conventional view, however, Applicants have discovered and successfully made a zeolite with little to no detectable amounts of a particular contaminant zeolite, namely, a LTA-type zeolite (hereinafter “LTA zeolite”). More specifically, Applicants have discovered and made a form of zeolite X with either little or no detectable LTA zeolite (“low LTA-containing X zeolite”), as determined by the x-ray diffraction (“XRD”) method described below, which also has a particle size not greater than 2.7 microns (μm), as determined by the sedigraph analysis described below. Applicants have also discovered that a low LTA-containing X zeolite is useful for making a zeolitic binder-converted composition (discussed below).
One adsorbent application of interest, among others, relates to separating para-xylene (pX) from a mixture of xylenes in a fixed bed process, which is often a simulated moving bed (SMB) adsorption process.
The SMB adsorption process is used commercially in a number of large scale petrochemical separations to recover high purity pX from mixed xylenes. As used herein, “mixed xylenes” refers to a mixture of C8 aromatic isomers that includes ethyl benzene (EB), pX, meta-xylene (mX) and ortho-xylene (oX). High purity pX is used for the production of polyester fibers, resins and films. Typically, pX is converted to terephthalic acid (TPA) or dimethyl terephthalate (DMT), which is then reacted with ethylene glycol to form polyethylene terephthalate (PET), the raw material for most polyesters.
The general technique employed in the performance of SMB adsorptive separation processes is widely described and practiced. Generally, the process simulates a moving bed of adsorbent with continuous counter-current flow of a liquid feed over the adsorbent. Feed and products enter and leave adsorbent beds continuously, at nearly constant compositions. Separation is accomplished by exploiting the differences in affinity of the adsorbent for pX relative to the other C8 aromatic isomers.
Typical adsorbents used in SMB adsorption processes generally include crystalline aluminosilicate zeolites and can comprise both the natural and synthetic aluminosilicates. Suitable crystalline aluminosilicate zeolites for use as an adsorbent selective for pX include those having aluminosilicate cage structures in which alumina and silica tetrahedra are intimately connected with each other in an open three dimensional crystalline network. The tetrahedra are cross linked by shared oxygen atoms, with spaces between the tetrahedra occupied by water molecules prior to partial or total dehydration of the zeolite. The dehydration results in crystals interlaced with channels having molecular dimensions.
In a hydrated form the crystalline aluminosilicate zeolites are generally represented by the formula:M2/nO:Al2O3:wSiO2:yH2Owhere “M” is a cation that balances the electrovalence of the tetrahedra and is generally referred to as an exchangeable cationic site, “n” represents the valence of the cation, “w” represents the moles of SiO2, and “y” represents the moles of water. Such crystalline aluminosilicate zeolites that find use as an adsorbent possess relatively well-defined pore structures. The exact type aluminosilicate zeolite is generally identified by the particular silica: alumina molar ratio and the pore dimensions of the cage structures.
Cations (M) occupying exchangeable cationic sites in the zeolitic adsorbent may be replaced with other cations by ion exchange methods well known to those having ordinary skill in the field of crystalline aluminosilicates. Crystalline aluminosilicates, such as zeolite X with barium and potassium cations at the exchangeable cationic sites within the zeolite, are known to selectively adsorb pX in a mixture comprising at least one other C8 aromatic isomer beyond pX.
Generally, zeolitic adsorbents used in separation processes contain the zeolitic crystalline material dispersed in an amorphous material or inorganic matrix having channels and cavities that enable liquid access to the crystalline material. Silica, alumina or certain clays and mixtures thereof are typical of such inorganic matrix materials, which act as a “binder” to form or agglomerate the zeolitic crystalline particles that otherwise would comprise a fine powder. Agglomerated zeolitic adsorbents may thus be in the form of extrudates, aggregates, tablets, macrospheres such as beads, granules, or the like.
The binder is typically inert and contributes little, if any, to the adsorptive separation process. Efforts to improve adsorbent efficacy generally have focused on (a) decreasing the size of the zeolite particles forming the adsorbent and (b) increasing the zeolite volume (i.e., the active separation component) within adsorbents. One method for increasing the zeolite volume in the adsorbent is to convert the binder into zeolite in a conversion process referred to as “zeolitization,” while preferably maintaining or improving the adsorbent material's strength and macroporosity, among other things. This binder-conversion process thereby obtains a zeolitic binder-converted composition, which is often referred to as a “binderless” zeolitic adsorbent. However, the description of “binderless” does not necessarily mean all original binder material is converted to zeolitic material since some small fraction of binder material (e.g., up to 3 wt %) may not be converted, depending on various factors, such as, original binder content, zeolitization conditions, etc. While a binder-conversion process has resulted in improved adsorbent efficacy, still further improvements in adsorptive separation process efficiency are desired.
Accordingly, an improved zeolitic binder-converted adsorbent composition obtained from a X zeolite with improved purity—more specifically, a low LTA-containing X zeolite having a particle size not greater than 2.7 μm—to recover high purity pX from mixed xylenes in a liquid-phase separation process using the zeolitic binder-converted adsorbent is described more fully herein. A method for obtaining a low LTA-containing X zeolite having a particle size not greater than 2.7 μm is also described herein, as well as a method for obtaining a zeolitic binder-converted adsorbent using such a low LTA-containing X.
Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims.